Relaxation dynamics and acoustic properties in simple liquids

An overview of the relaxation dynamics in simple liquids is undertaken, in view of the recent experimental results achieved by means of inelastic X-ray and neutron scattering. We address some basic issues related to the extension of ordinary hydrodynamic in the Thz domain, such as the coupling of density and entropy fluctuations and the origin of anomalous acoustic dispersion within the general context of disordered systems dynamics. Specifically, we show that the Landau Placzeck ratio do not describe the quasielastic component of the dynamic structure factor and that the positive dispersion is a universal feature which can be understood in terms of a disorder induced relaxation process.